From Layers to Nanotubes: Transition Metal Disulfides TMS2

MoS2 and WS2 layered transition-metal dichalcogenides are indirect band gap semiconductors in their bulk forms. Thinned to a monolayer, they undergo a transition and become direct band gap materials. Layered structures of that kind can be folded to form nanotubes. We present here the electronic structure comparison between bulk, monolayered and tubular forms of transition metal disulfides using first-principle calculations. Our results show that armchair nanotubes remain indirect gap semiconductors, similar to the bulk system, while the zigzag nanotubes, like a monolayer, are direct gap materials, what suggests interesting potential applications in optoelectronics.Comment: published in EPJ B, 9 pages, 8 figure

Structure-Electronic Property Relationships of 2D Ruddlesden-Popper Tin- And Lead-based Iodide Perovskites

Two-dimensional (2D) halide perovskites are receiving considerable attention for applications in photovoltaics, largely due to their versatile composition and superior environmental stability over three-dimensional (3D) perovskites, but show much lower power conversion efficiencies. Hence, further understanding of the structure-property relationships of these 2D materials is crucial for improving their photovoltaic performance. Here, we investigate by means of first-principles calculations the structural and electronic properties of 2D lead and tin Ruddlesden-Popper perovskites with general formula (BA)2An-1BnI3n+1, where BA is the butylammonium organic spacer, A is either methylammonium (MA) or formamidinium (FA) cations, B represents Sn or Pb atoms, and n is the number of layers (n = 1, 2, 3, and 4). We show that the band gap progressively increases as the number of layers decreases in both Sn- and Pb-based materials. Through substituting MA by FA cations, the band gap slightly opens in the Sn systems and narrows in the Pb systems. The electron and hole carriers show small effective masses, which are lower than those of the corresponding 3D perovskites, suggesting high carrier mobilities. The structural distortion associated with the orientation of the MA or FA cations in the inorganic layers is found to be the driving force for the induced Rashba spin-splitting bands in the systems with more than one layer. From band alignment diagrams, the transfer process of the charge carriers in the 2D perovskites is found to be from smaller to higher number of layers n for electrons and oppositely for holes, in excellent agreement with experimental studies. We also find that, when interfaced with 3D analogues, the 2D perovskites could function as hole transport materials.</p

Expression de l'Annexine A2et partenaires pendant la différenciation des cellules épithéliales

International audienceThe members of the annexin family of calcium-and phospholipid-binding proteins participate in different cellular processes. Annexin A2 binds to S100A10 forming a functional heterotetrameric protein that has been involved in many cellular functions such as exocytosis, endocytosis, cell junction formation and actin cytoskeleton dynamics. Herein, we studied annexin A2 cellular movements and looked for its partners during epithelial cell differentiation. By using immunofluorescence, mass spectrometry and western blot analyses after S100A10 affinity column separation, we identified several annexin A2-S100A10 partner candidates. The association of putative annexin A2-S100A10 partner candidates obtained by MS after column affinity was validated by immunofluorescence and sucrose density gradient separation. The results show that three proteins were clearly associated to AnxA2: E-cadherin, actin and caveolin 1. Overall the data show that annexin A2 is able to associate to molecular complexes containing actin, caveolin 1 and flotillin 2 before epithelial differentiation and to complexes containing E-Cadherin, actin and caveolin 1, but not flotillin 2 after cell differentiation. The results indicate that actin, caveolin 1 and E-cadherin are the principal 2 protein partners of annexin A2 in epithelial cells and that the serine phosphorylation of the N-terminal domain does not play an essential role during epithelial cell differentiation.Les membres de la famille des annexines, des protéines liant le calcium et les phospholipides, participent à différentsprocessus cellulaires. L’annexine A2 se lie à S100A10, formant une protéine hétérotétramérique fonctionnelle qui a été impliquéedans plusieurs fonctions cellulaires telles que l’exocytose, l’endocytose, la formation des jonctions cellulaires et la dynamique ducytosquelette. Les auteurs ont étudié ici les mouvements cellulaires de l’annexine A2 et recherché ses partenaires durant ladifférenciation des cellules épithéliales. À l’aide de l’immunofluorescence, de la spectrométrie de masse et des analyses detransferts « western » après séparation sur une colonne d’affinité S100A10, ils ont identifié plusieurs candidats partenaires del’annexine A2–S100A10. L’association de candidats partenaires présumés de l’annexine A2–S100A10 obtenus par SM après lacolonne d’affinité a été validée par immunofluorescence et séparation sur un gradient de densité de sucrose. Les résultatsmontrent que trois protéines sont clairement associées à l’annexine A2 : la E-cadhérine, l’actine et la cavéoline 1. Globalement,les données montrent que l’annexine A2 peut s’associer à des complexes moléculaires comprenant l’actine, la cavéoline et laflotilline 2 avant la différenciation épithéliale et à des complexes comprenant la E-cadhérine, l’actine et la cavéoline 1, mais pasla flotilline 2, après la différenciation cellulaire. Les résultats indiquent que l’actine, la cavéoline et la E-cadhérine constituent lesprincipaux partenaires protéiques de l’annexine A2 dans les cellules épithéliales et la phosphorylation des sérines du domaineN-terminal ne joue pas un rôle essentiel durant la différenciation des cellules épithéliales

Two dimensional crystals in three dimensions: electronic decoupling of single-layered platelets in colloidal nanoparticles

Two-dimensional crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spinorbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, we propose the formation of stacks of two-dimensional crystals intercalated with small surfactant molecules. We show, using first principles calculations, that already the very short surfactant methyl amine electronically decouples the layers. We demonstrate the indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the two-dimensional crystals with electronic properties typical for monolayers. Variation of the surfactant's chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects

Synthesis and Rheological Properties of Magnetic Chitosan Hydrogel

The aim of the present work is first to synthesis a magnetic chitosan hydrogel (chitosan ferrogel) using the blending method and second to study it rheological behavior. Magnetic components ( maghemite particles γ-Fe2O3 ) were synthesized via a simple chemical co-precipitation route also called Massart's procedure. Before being dispersed in chitosan network, γ-Fe2O3 particles were covered with a cationic polyelectrolyte (Polydiallyldimethylammonium chloride; PDADMAC) and the exact quantity required to cover the entire surface of maghemite particles was determined by Electrophoretic mobility. The successful functionalization of maghemite particles was confirmed by zeta potential measurement. The prepared ferrogel was gelified using glyoxal as crosslinking agent. The effect of continuous magnetic field on rheological properties of the elaborated ferrogel was studied, under controlled temperature before and after the gelation process, using a rotating rheometer fitted with a new magneto-rheological cell. Moreover the influence of iron oxide content on the gelation time of magnetic hydrogel was studied by comparing two ferrogels with different maghemite particles content. Flow and viscoelastic measurements showed that applying magnetic field facilitates the formation of a new structure (column-like arrangements), which was confirmed by in situ optical microscopic observation. Kinetic study was investigated by mechanical spectroscopy and demonstrates that the gelation time depends on both iron oxides content and magnetic field